Measurement of the energy spectrum of the cosmic rays at the highest energies using the 

Pierre Auger Observatory

Alejandra Parra

For the Pierre Auger Collaboration*

University of Santiago de Compostela (Spain)

*Full author list:

http://www.auger.org/archive/authors_2013_01.html

Physical CaseEnergy Spectrum analysis of special interest at:

E ~ 4 EeV - The Ankle– Ankle model.– Dip model.– Mixed composition

model. E >~ 50 EeV –

Flux suppression.– GZK effect– Source effects.

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Pierre Auger Observatoy (PAO)Hybrid detectionLocated at Malargüe, (Argentina.)Surface Detector (SD)– 1660 water-Cherenkov stations.– Hexagonal grid of 1.5 km spacing– Covering an area of 3000km2– Low energy extension: Infill (61 stations

in 0.75 km spacing grid)

Fluorescence Detector (FD):– 24 fluorescence telescopes in 4 buildings

overlooking SD– Low energy extension: HEAT (3 telescopes

overlooking the Infill) Taking data from 2004 (completed in

2008)

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Hybrid detectorSimultaneous detection of extensive air showersTwo complementary techniques:– Lateral profile measured with SD (100% duty cicle)– Longitudinal profile recorded with FD (~10% duty cicle)

Different shower parameters contribute to identify the primary arrival direction and energy.

Accurace measurements and cross-checks.

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Measurement of the UHECRs flux with SD and FD

SD Vertical (Θ < 60°)Full efficiency at 3 x 1018 eVGeometrical acceptance (based

on the operational tanks configuration)

Two different analysis for SD events according the zenith angle: Vertical (Θ < 60°) and Inclined (62°< Θ < 80°)

SD inclined (62° < Θ < 80°) Full efficiency at 4 x 1018 eVGeometrical acceptance

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SD Infill Array (Θ < 55°)Full efficiency at 3 x 1017 eVGeometrical acceptance (preliminary)

SD Infill inclined (under work)

Hybrid (FD + 1 SD station) Full efficiency at 1018 eVAcceptance from simulations

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FD Reconstruction of the Shower Energy

FD Calorimetric energy measurement.

FD info

Atmosphere as an inhomogeneous calorimeter.

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SD Energy EstimatorsVertical Shower (Θ < 60°)

θ

N19(E,A) = Nµ/Nµ(1019 eV, p,θ)

Inclined Shower (Θ > 60°)S38= S1000/CIC(Θ)

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SD Energy Calibration with FDHigh quality hybrid events are used to calibrate the SD energy estimator with the FD calorimetric energy

ESD = A · SB38A = (1.68± 0.05)x1017 eVB = 1.035± 0.009Systematic uncertainties:

7% (15%) at 10 EeV (100 EeV)

ESD = A · NB19A = (4.69± 0.09)x1018 eVB = 1.05± 0.02Systematic uncertainties:

13% (50%) at 10 EeV (80 EeV)

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SD Energy ResolutionFor θ < 60º : Energy resolution obtained from golden hybrids

– (16±1)% for 3 EeV < E < 6 EeV– (13±1)% for 6 EeV < E < 10 EeV

For θ > 62°: Energy resolution otained from simulations.

– From 25% at 3EV up to 12% at high energies.

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Energy Spectrum with Hybrid Events

Hybrid events selected with at least one SD station with signal

Full efficiency above 1018 eV (ankle region)Energy resolution σ(E)/E=7.6%Exposure calculation from detector simulations tacking in account its time evolution (atmospheric conditions, detector configuration, data taking)

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Systematic UncertaintiesEnergy Systematic Uncertainties:– Fluorescence yield 14%– Reconstruction 10%– Absolute calibration

9.5 %– Atmospheric effects

8%– Invisible energy 4%

Total uncertainty 22%

Flux systematic uncertainties:– SD vertical 6%

• Exposure 3%• Forward Folding 5%

– SD inclined 13% up to 40 EeV(exposure + calibration)

– Hybrid 10% (6%) at 1 EeV (10 EeV)• Hadronic interaction

models < 2%• Mass composition

assumption:– 1 EeV 8%– 10 EeV 1%

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Exposure Comparison

Exposures at 10 EeV:– Vertical 20905 km2 sr year– Hybrid 885 km2 sr year– Inclined 5600 km2 sr year– Infill 26 km2 sr year

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Different measurements of the energy spectrum

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Combined Energy Spectrum

Spectra in very good agreement (better than 1.5%)Deconvolution of the inclined spectrum for the energy resolution effects in progress.

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Combined spectrum fitted to three power laws with free breaks between them.

Precise measurement of the spectral features. Significance of the flux suppression: >20 sigma.

Spectral Features

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ConclusionsFour different measurements of the UHECR flux with the Pierre Auger Observatory, having in common only the energy scale.

These approaches give compatible results over all the energy range.

Extend the energy range down (1017 eV) with the Infill Array and High Elevation Auger Telescope (HEAT)

Reduce the systematic uncertainties on the energy(reconstruction improvements, fluorescence yield contribution).

Update of the spectra in the next ICRC 2013

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